This invention relates to mechanism for maintaining a predetermined tautness in a conventional drive belt interposed between drive and driven pulley elements. Previous belt tension (tautness) devices known to applicant utilize springs having positive spring rates. When such springs are used the belt tension device delivers the required or specified force only at one deflected condition. If the device is adjusted to apply proper belt tension force when the belt is first installed then after a period of time any belt stretch or wear will cause the device to deliver an insufficient force to take up belt slack. The device must periodically be readjusted manually.
In practice new drive belts are often installed with belt-tension device set to slightly overtension the belt; as the belt wears and elongates the tension is automatically reduced to the desired value, and then later to an undesirably low value. For example, if the "proper" tension is eight pounds the belt-tension device may be initially set at ten pounds; when the belt stretches a sufficient amount the tension drops down to the desired eight pounds. Unfortunately this eight pound condition occurs for only a relatively brief period; usually the tension continues to decrease to a lower value, for example six pounds, thereby tending to promote belt slippage and frictional wear.
In the present invention a zero rate constant force spring provides the belt-tension force. Therefore the device can be initially set at the desired value, for example eight pounds, with the assurance that it will remain at the initial force setting even though the belt may elongate over a period of time.